1. Field of the Invention
This invention relates to gas turbine engines, and more specifically to an apparatus for reducing nitric oxide (NOx) emissions and cooling the combustion liner for a can-annular gas turbine combustion system.
2. Description of Related Art
Combustion liners are commonly used within the combustion section for most gas turbine engines. They serve to protect the combustor casing and surrounding engine from the extremely high operating temperatures by containing the chemical reaction that occurs between the fuel and air.
Recently, government emission regulations have become of great concern to both gas turbine manufacturers and operators. Of specific concern is emission of nitric oxide (NOx) and its contribution to air pollution, since utility sites have governmental permits that restrict allowable amounts of NOx emissions per year. It is therefore desirable to have engines with lower emission rates, especially NOx, since these engines are allowed to run longer hours and, as a result, generate more revenue for their operators.
It is well known that NOx formation is a function of flame temperature, air inlet temperature, residence time, and fuel/air ratio. Lower flame temperature, shorter residence time, and lower fuel/air ratio have all been found to lower NOx emissions. Lower flame temperature and lower fuel/air ratios can be achieved by increasing the amount of air introduced in the combustion process, for a given amount of fuel.
However, due to the high operating temperatures of gas turbines, a significant portion of the air exiting the engine""s compressor is needed to cool the engine parts to prevent their premature failure. Since much of the air used for cooling such parts bypasses the combustor, increases in cooling air demands reduce the air available for combustion, thereby increasing the fuel/air ratio for a given fuel flow, resulting in a higher flame temperature that tends to exacerbate NOx emission problems.
What is needed is an apparatus that maximizes the use of available air for combustion by using the air originally dedicated only for cooling to lower the combustor""s fuel/air ratio, and in turn, lowering NOx emissions.
It is therefore an object of the present invention to provide a means to increase the amount of air to the combustion zone of a dual stage dual mode combustion liner.
It is a further object of the present invention to provide an aid for assembly and disassembly of the combustion liner to the combustion system.
According to the present invention a combustion liner intended for use in a dry, low NOx gas turbine engine, of the type typically used to drive electrical generators is disclosed. The combustion liner includes an upstream premix fuel/air chamber and a downstream, or secondary, combustion chamber, separated by a venturi having a narrow throat constriction. A plenum is utilized to direct cooling air from the venturi into the premix chamber, which in turn, reduces the level of NOx emissions. Depending on the size of the combustion liner and its mating hardware, typically a flow sleeve or heatshield, the size of the air plenum, and hence the amount of air that can be introduced into the premix chamber, is limited. The invention disclosed in this application helps to overcome this limitation by introducing a structural insert, or recess, to the air plenum that aids in assembly of the combustion liner to the mating hardware, when the air plenum is oversized.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.